When, in general, tin oxide film is formed on the surface of a glass substrate, the tin oxide film is found to have an excellent hardness, so that it is in wide use for the prevention of the formation of flaws on the surfaces of glassware such as bottles, tableware, electric bulbs, etc. Also, there are many applications utilizing the fact that the tin oxide film doped with a halogen, especially fluorine, or antimony, exhibits metallic properties. For instance, glasses on whose surface tin oxide film has been formed, are in use as transparent electrode plates of semiconductor devices including liquid crystal cells, solar cells, etc., as well as the surface heating unit of antifogging glass, etc. Further, the tin oxide film is also extremely useful for window glass used for the purpose of improving air heating or as the cover glass of solar heat collector, by virtue of its infrared ray reflection characteristics.
Heretofore, as the process for coating a glass substrate with tin oxide, various processes have been proposed, but as the process most adapted for mass production, processes taking advantage of a thermal decomposition oxidation reaction of tin compounds are known. For instance, from long ago a process has been used wherein a solution containing tin tetrachloride dissolved in a solvent is sprayed on the high temperature surface of a glass substrate (liquid spraying process) or a process wherein a fine powder of organotin compounds such as dibutyltin oxide, etc. suspended in a gas is sprayed on the high temperature surface of a glass substrate (powder spraying process). Recently, the so-called CVD process wherein the vapor obtained by evaporating tin tetrachloride or dimethyltin dichloride is brought into contact with the high temperature surface of a glass substrate is also in wide use. For the tin oxide films formed by these processes, the fields of their application have recently been rapidly increasing, particularly, wherein their high electrical conductivity and high light transmittance are demanded. For this reason, there has been proposed for the purpose of obtaining a tin oxide film having the high electrical conductivity a process wherein use is made of a novel tin compound, and a process characterized in that a suitable amount of an additive is mixed with a tin compound. For instance, a process wherein a tin oxide film excellent in electrical conductivity is obtained by the use of a vapor mixture of tetramethyltin [(CH.sub.3).sub.4 Sn] and bromotrifluoromethane [CF.sub.3 Br] according to a CVD (chemical vapor deposition) process has been described in Japanese Patent Application (OPI) No. 58363/80 (the term "OPI" as used herein means an "unexamined published application"). Also, a process wherein a tin oxide film excellent in the electrical conductivity is obtained by the use of a vapor mixture of monobutyltin trichloride [C.sub.4 H.sub.9 SnCl.sub.3 ] and a fluorine-containing compound of the structure CF.sub.2 HX (X is hydrogen, halogen, or alkyl radical) according to a CVD process has been described in Japanese Patent Application (OPI) No. 162269/84.
In addition to the above, a process is also known wherein quartz glass or borosilicate glass is used in place of soda-lime glass as the substrate for the purpose of obtaining a tin oxide film excellent in the electrical conductivity, or in the case where the substrate is soda-lime glass, a process wherein the surface of the substrate is preliminarily covered with silicon oxide film prior to the formation of the tin oxide film.
However, the electrical conductivity of the tin oxide films obtained according to these processes is such that the electrical conductivity, .sigma.=3000.OMEGA.-1cm-1 (specific resistance .rho.=1/.sigma.=3.3.times.10.sup.-4 .OMEGA..multidot.cm) is the limit attainable, showing that it is uncomparable to that of the tin oxide-added indium oxide film.